PLDM: Mark all _current attributes as readonly

- The _current attributes are supposed to be only set by HOST and
NOT by any redfish client, so by marking them as readonly
we would deny customers/redfish client to do PATCH operations
on _current attributes.

- List of attribute modified as readOnly:
hb_cap_freq_mhz_request_current
hb_field_core_override_current
hb_host_usb_enablement_current
hb_huge_page_size_current
hb_key_clear_request_current
hb_lateral_cast_out_mode_current
hb_memory_mirror_mode_current
hb_memory_region_size_current
hb_mfg_flags_current
hb_number_huge_pages_current
hb_power_limit_enable_current
hb_proc_favor_aggressive_prefetch_current
hb_tpm_required_current
pvm_create_default_lpar_current
pvm_default_os_type_current
pvm_os_boot_type_current
pvm_rpa_boot_mode_current
pvm_stop_at_standby_current
pvm_vtpm_current

Tested:
- Tried to change attribute using redfish command which is blocked now.
- Tried to change attribute using pldmtool command which is working fine.

Change-Id: I879266d73e033dd55ff6c61e414bea2fd617bbef
Signed-off-by: Patel-Kamalkumar <kamalkumar.patel@ibm.com>
diff --git a/oem/ibm/configurations/bios/string_attrs.json b/oem/ibm/configurations/bios/string_attrs.json
index a91ff6f..37a4935 100644
--- a/oem/ibm/configurations/bios/string_attrs.json
+++ b/oem/ibm/configurations/bios/string_attrs.json
@@ -124,7 +124,8 @@
             "default_string_length": 32,
             "default_string": "00000000000000000000000000000000",
             "helpText": "Specifies the configuration flags used by manufacturing for the current IPL. Do not set this attribute directly; set hb_mfg_flags instead.",
-            "displayName": "Manufacturing Flags (current)"
+            "displayName": "Manufacturing Flags (current)",
+            "readOnly": true
         },
         {
             "attribute_name": "hb_lid_ids",